It relates to an outer loop transmission power control method used in mobile communication systems, and to calculation of a transmission power control target value.
In conventional mobile communication systems such as 3GPP systems, inner loop transmission power control and outer loop transmission power control are performed as transmission power control of radio waves traveling in a direction (uplink direction) from a mobile terminal (mobile station (MS) or user equipment (UE)) to a base station (base transceiver station (BTS) or node B).
In the inner loop transmission power control, the following operation is generally performed. For example, the base station measures a signal-to-interference ratio (SIR: target-received-wave-to-interference-wave ratio) of uplink-direction radio waves received from the mobile terminal, and compares the measured value with a predetermined value as a target SIR. Based on the comparison result, the base station determines to increase or reduce transmission power of radio waves to be transmitted by the mobile terminal, and sends power control information (called a transmission power control (TPC) command) indicating an increase or reduction in transmission power, to the mobile station. For example, when the measured SIR is lower than the target SIR, a TPC command indicating an increase in transmission power is sent. The mobile station sends, to the base station, radio waves with the transmission power controlled based on the TPC command. A feedback loop formed between the base station and the mobile station in this way is called an inner loop.
On the other hand, in the outer loop transmission power control, the target SIR to be used for the above-described inner loop transmission power control is controlled. For example, a base station control apparatus (base station controller (BSC) or radio network controller (RNC)) which accommodates the base station, which performs the inner loop transmission power control, obtains information (data) acquired from radio waves received by the base station, from the base station, and determines the quality of the data. At this time, when the quality does not reach the target quality, the base station control apparatus determines a target SIR so as to improve the quality, and notifies the target SIR to the base station. A feedback loop formed between the base station and the base station control apparatus in this way is called an outer loop.
As described above, in the outer loop transmission power control, the target SIR, which is one of parameters for uplink data sent from the mobile terminal, is controlled. The target SIR indicates the target reception quality of data to be received by the base station from the mobile terminal. When data sent from the mobile terminal is received by the base station control apparatus through the base station, quality information of the radio section is attached on the data. The base station control apparatus measures the quality of the data based on the quality information for a given period of time. When the given period of time has elapsed, the base station control apparatus calculates an uplink-direction target SIR to be used in the inner loop, based on measurement results. When the calculated target SIR is different from the last target SIR which has been specified in the base station, the base station control apparatus notifies the target SIR calculated this time to the base station. The base station updates the target SIR.
Diversity hand over (DHO) among base stations in the currently-used 3GPP systems (third generation mobile communication systems) is realized by the use of multiple radio transmission paths, and selection/compounding and multiple distributing performed in the mobile terminal and the base station control apparatus.
For example, as shown in FIG. 1, it is assumed that DHO is performed between the mobile terminal (UE) and the base station control apparatus (RNC) through three base stations (three node B). When data transmission and reception are performed in a downlink direction (RNC to UE), the RNC sends identical data to the base stations through interfaces Iub. The base stations modulate the data by using different spreading code, and send the modulated data to the UE through different radio-section interfaces Uu (radio transmission paths). Among the pieces of data sent from the base stations, the UE selects data having good radio transmission path quality (data arrived without an error) and receives the selected data. On the other hand, when data transmission and reception are performed in an uplink direction (UE to RNC), the UE sends identical data to the base stations. The base stations send the data received from the UE, to the RNC. The RNC selects data having good radio transmission path quality among the pieces of data received from the base stations, or collects parts of the data having good quality from the pieces of data received from the base stations and compounds the parts. With this method, a diversity effect can be obtained between the UE and the RNC.
In the above-described systems, identical data is sent to the multiple radio transmission paths. Therefore, when the number of radio transmission paths is increased, the available radio capacity is reduced.
Further, the pieces of data (signals) sent from the base stations to the mobile terminal through the radio transmission paths are modulated by using the different spreading codes. Therefore, when the mobile terminal demodulates a signal sent through a certain radio transmission path by inverse-spreading, noise is produced in a signal sent through another radio transmission path (see FIG. 2). Therefore, each of the base stations needs to increase the transmission power in order to assure the desired SIR.
Further, in the above-described diversity reception system employed in the RNC, it is necessary to receive data with no error from at least one of the multiple radio transmission paths in order to select and combine pieces of data sent from multiple base stations. For this reason, a minimum target SIR for obtaining the diversity effect is specified in each base station.
As described above, there is a problem in that when DHO is performed among base stations in conventional 3GPP systems, the available radio capacity (radio resources) or transmission power cannot be efficiently used.
As a technology of avoiding the problem regarding the use of radio resources, for example, Patent Document 1 discloses a radio communication system and a handoff method therefor. In the radio communication system, handoff means is provided which maintains a consistent amount of used downlink radio resources in a radio base station regardless of whether a handoff is being made or not.    Patent Document 1: JP 2000-217139 A